The IEEE 802.16e system is part of a global standardization for broadband wireless access systems. IEEE 802.16e does not have a hierarchical structure of HLR, VLR, MSC, BSC, RNC, etc. unlike second and third generation mobile communications systems, but does include a mobile subscriber station (MSS), a base station (BS), and an authentication service authorization server (ASA server). A physical (PHY) layer and a medium access control (MAC) layer are defined between the BS and the MSS.
A handover process in a broadband wireless access system according to a related art is explained as follows.
1. Pre-Processing for Handover
Pre-processing for handover includes broadcasting associated information from a base station to a neighboring base station to inform all mobile subscriber stations within a respective cell of the associated information (Network Topology Advertisement). The pre-processing also includes measuring a channel quality of the neighboring base station based on the associated information (MSS Scanning of Neighboring BS). The pre-processing also includes matching parameters between the base station and the neighboring base station, such as for example, an initial mobile subscriber station power, a time difference for synchronization, and other parameters (Association Procedures).
a. Network Topology Advertisement
In network topology advertisement, the base station broadcasts information associated with network configuration to all mobile subscriber stations within a cell via a MOB_NBR-ADV MAC message, thereby providing all mobile subscriber stations within the cell with the information pertaining to neighboring base stations.
b. MSS Scanning of Neighboring BS
In MSS scanning of neighboring BS, a mobile subscriber station (MSS) scans a neighboring base station (BS) and requests a scanning interval for scanning the neighboring BS from the BS via a MOB_SCN-REQ MAC message. The BS then transmits a MOB_SCN-RSP MAC message in response to the request to assign an interval for scanning (scanning interval) the neighboring BS. Alternatively, the BS may directly transmit the MOB_SCN_RSP MAC message without a request from the MSS (unsolicited response). In the case of an unsolicited response, the scanning interval and an offset unit for scanning start are assigned on a frame unit basis.
c. Association Procedures
During association procedures, the MSS performs a ranging process with a BS to normally join a cell. In the ranging process, the MSS acquires basic information for communications from the BS. In other words, the MSS performs BS scanning to select a new BS and then performs the association procedures. The MSS and the BS transmit a RNG_REQ MAC message and a RNG_RSP MAC message, respectively, to set values such as for example, a power offset value, a timing offset value, and other values of the MSS to suitable values. Transmission of the RNG_REQ MAC message is called an initial ranging operation and is a basic operation that the MSS may perform in a network entry procedure. When a handover is performed, a target BS accepts a new MSS and transmits items associated with the respective cell of the new MSS to the former serving BS and stores information pertaining to the MSS.
2. Handover Process
In a handover (HO) process, the MSS initiates handover with the neighboring BS based on the channel quality information acquired in the handover pre-processing.
a. Cell Selection
In a cell selection procedure, a cell is changed to enable the MSS to make a new registration to a BS, allowing for reception of a signal having an SINR (signal to interference noise ratio) superior to that of a signal transmitted from the BS of the current cell, prior to a normal registration of the MSS to a cell. In such case, since the MSS does not perform the registration procedure, the BS is unable to determine a moving situation of the MSS.
b. HO (Handover) Initiation
In HO initiation, a handover may be initiated by the MSS or the BS. When the BS requests (initiates) a handover, the BS transmits a MOB_BSHO-REQ MAC message to the MSS. On the contrary, when the MSS requests a handover, the MSS transmits a MOB_MSSHO-REQ MAC message to the BS.
If the MSS transmits the MOB_MSSHO-REQ MAC message, the MSS preferentially transmits the SINR of a signal received from a neighboring BS to the BS to enable the MOB_MSSHO-REQ MAC message to be transmitted to a current serving BS. The current serving BS then selects a candidate BS as a target BS for performing the handover.
In a case where the BS receives the MOB_MSSHO-REQ MAC message from the MSS or in a case when the BS has not yet transmitted the MOB_BSHO-REQ MAC message to handover the MSS, the BS may allow the handover of the MSS after confirmation of an acknowledgement (ACK) for handing over a specific MSS from the neighboring BS. After receiving the MOB_BSHO/MSSHO-REQ MAC message, the MSS/BS transmits a MOB_MSSHO/BSHO-RSP MAC message to inform the BS/MSS of a target BS for the handover.
c. HO Cancellation
In HO cancellation, after the MSS/BS transmits the MOB_MSSHO/BSHO-REQ MAC message, the MSS may cancel the handover. In canceling the HO, the MSS sets a specific field (HO_Type=01) of a MOB_HO-IND MAC message and transmits the set field to the BS. The BS receives the message and cancels the proceeding handover.
d. Termination with the Serving BS
In termination with the serving BS, the MSS transmits the MOB_HO-IND MAC message to inform the serving BS that the handover is normally completed and then completes the handover operation. In completing the handover operation, the MSS sets a specific field (HO_Type=00) of the MOB_HO-IND MAC message to transmit the information pertaining to the normal handover termination to the serving BS. The serving BS receives the MOB_HO-IND MAC message from the MSS and terminates a MAC state machine allocated to the MSS. The serving BS also terminates an ARQ (automatic repeat request) connection and all connections associated with data transmission.
e. HO Rejection
In HO rejection, the MSS is capable of rejecting a handover recommended by the BS. In rejecting a handover, the MSS sets a specific field (HO_Type=10) of the MOB_HO-IND MAC message and transmits the message to the BS. The BS receives the rejection message from the MSS, re-seeks for a target BS and re-transmits the MOB_BSHO-RSP MAC message to the MSS.
FIG. 1 is a diagram illustrating a handover process requested by a mobile subscriber station (MSS).
Referring to FIG. 1, an MSS 11 performs a scanning procedure to measure qualities of signals transmitted from neighboring BSs (S110). If the MSS 11 determines that a handover needs to be performed based on results of the neighboring BS scanning procedure, the MSS 11 transmits a handover request message (MOB-MSS-HO-REQ) to a serving BS 12 (S111).
The serving BS 12 receives the handover request message and transmits a handover pre-notification message (HO-pre-notification) including information that the MSS 11 has requested a handover to a neighboring BS, such as for example, handover target BSs (target BSs) 13 and 14 (S112, S113). The handover pre-notification message includes an identifier (MSS identifier) of the MSS 11 requesting the handover, connection parameters, capabilities, a required bandwidth, and quality of service information.
Each of the neighboring handover target BSs 13 and 14 receives the handover pre-notification message and transmits a handover pre-notification response message (HO-pre-notification-response) to the serving BS 12 in response to the handover pre-notification message (S114, S115). The handover pre-notification response message may include acknowledgement information pertaining to the handover pre-notification message and capability information pertaining to the requested quality of service.
The serving BS 12 selects one of the neighboring handover target BSs 13 and 14 according to the response messages for the handover pre-notification and transmits a handover confirm message (HO-confirm) to the selected target BS 13 or 14 (S116). The serving BS 12 then transmits a handover response message (MOB-BSHO-RSP), including information pertaining to the selected handover target BS, to the MSS 11 (S117). The MSS 11 receives the handover response message and transmits a handover indicator message (MOB-HO-IND) to release a connection from the MSS 11 to the serving BS 12 (S118). Upon receipt of the handover indicator message from the MSS 11, the serving BS 12 releases its connection to the MSS 11 (S119).
The MSS 11 then executes a fast ranging (Fast Ranging_IE), a ranging request (RNG-REQ), and a ranging response (RNG-RSP) with the target BS 14 (S120, S121, S122). The target BS 14 becomes the serving BS 14 and the MSS 11 completes an initial network entry execution after handover (S123).
FIG. 2 is a diagram illustrating a handover process requested by a base station (BS).
Referring to FIG. 2, a serving BS 22 triggers a procedure (trigger-to-release-MSS) to release a connection from an MSS 21 according to a corresponding decision (S210).
The serving BS 22 then transmits a handover pre-notification message (HO-pre-notification) including information required for the handover of the MSS 21 to neighboring handover target BSs 23 and 24 (S211, S212). The handover pre-notification message includes an identifier (MSS identifier) of the MSS 21 requesting the handover, connection parameters, capabilities, a required bandwidth, and quality of service information.
Each of the neighboring handover target BSs 23 and 24 receive the handover pre-notification message and transmit a handover pre-notification response message (HO-pre-notification-response) to the serving BS 22 in response to the handover pre-notification message (S213, S214). The handover pre-notification response message may include acknowledgement information pertaining to the handover pre-notification message and capability information pertaining to the requested quality of service.
The serving BS 22 then transmits a handover request message to the MSS 21 based on the information received via the handover pre-notification response message (S215). The handover request message includes information pertaining to the target BSs 23 and 24 available for the handover. The MSS 21 then performs a neighboring BS scanning procedure to measure quality of signals received by the neighboring target BSs, as recognized via a handover response message (S216). In a case where the target BS 24 is found for the handover is found as a result of the neighboring BS scanning, the MSS 21 transmits a handover response message including target BS information for the handover to the serving BS 22 (S217). The serving BS 22 then transmits a handover confirm message to the target BS 24 to which the MSS 21 intends to perform the handover (S218).
The MSS 21 then transmits a handover indicator message (MOB-HO-IND) for releasing a connection from the MSS 21 to the serving BS 22 (S219). Upon receipt of the handover indicator message, the serving BS 22 releases its connection to the MSS 21 (S220).
The MSS 21 then executes a fast ranging (Fast Ranging_IE), a ranging request (RNG-REQ), and a ranging response (RNG-RSP) (S221, S222, S223) with the serving target BS 24 for the handover and then completes an initial network entry execution after handover (S224).
FIG. 3 is a diagram illustrating a process by which an MSS re-enters a former serving BS, such as in the case of a call drop.
Referring to FIG. 3, an MSS 31 performs a scanning procedure to measure qualities of signals transmitted from neighboring BSs (S310). If the MSS determines that a handover needs to be performed based on results of the neighboring BS scanning procedure, the MSS 31 transmits a handover request message (MOB-MSS-HO-REQ) to a serving BS 32 (S311).
The serving BS 32 receives the handover request message and transmits a handover pre-notification message (HO-pre-notification) including information that the MSS 31 has requested a handover to a neighboring BS, such as for example, handover target BSs 33 and 34 (S312, S313). The handover pre-notification message includes an identifier (MSS identifier) of the MSS requesting the handover, connection parameters, capabilities, a required bandwidth, and quality of service information.
Each of the neighboring handover target BSs 33 and 34 receive the pre-notification message and transmit a handover pre-notification response message (HO-pre-notification-response) to the serving BS 32 in response to the handover pre-notification message (S314, S315). The handover pre-notification response message may include acknowledgement information pertaining to the handover pre-notification message and capability information pertaining to the requested quality of service.
The serving BS 32 selects one of the neighboring handover target BSs 33 and 34 according to the response messages for the handover pre-notification and transmits a handover confirm message (HO-confirm) to the selected target BS 33 or 34 (S316). The serving BS 32 then transmits a handover response message (MOB-BSHO-RSP) including information pertaining to the target BS for the handover to the MSS 31 (S317). The MSS 31 receives the handover response message from the serving BS 32 and transmits a handover indicator message (MOB-HO-IND) to release a connection from the MSS 31 to the serving BS 32 (S318). Upon receipt of the handover indicator message, the serving BS 32 releases its connection to the MSS 31 (S319).